Insect in shirt, initially unseen (5)

I believe the answer is:

louse

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'insect' is the definition.
(I know that louse is a type of insect)

'in shirt initially unseen' is the wordplay.
'in' is an insertion indicator.
'shirt' becomes 'lose' (I can't justify this - if you can you should believe this answer much more).
'initially' indicates taking the first letters.
The initial letter of 'unseen' is 'u'.
'lose' going around 'u' is 'LOUSE'.

Can you help me to learn more?

(Other definitions for louse that I've seen before include "Wingless parasite" , "Wingless bloodsucking insect" , "Unpleasant person" , "Bloodsucking parasitic insect" , "25 [PARASITE]" .)

I've seen this clue in The Telegraph.

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